A MTJ element in a magnetic device such as a read head may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. The bottom layer in the MTJ element is typically comprised of a seed layer such as NiFeCr or a Ta/NiCr composite which promotes a <111> lattice orientation in overlying layers. Generally, an antiferromagnetic (AFM) layer, ferromagnetic “pinned” layer, tunnel barrier layer, ferromagnetic “free layer”, and a capping layer are successively formed on the seed layer to complete the MTJ stack. The pinned layer has a magnetic moment that is fixed in the “x” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “x” direction. The thin tunnel barrier layer above the pinned layer is usually comprised of a dielectric material such as AlOx or MgO and is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed through the MTJ stack in a direction perpendicular to the layers therein, a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
In an MRAM device, the MTJ element is formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line. In a read operation, the information stored in an MRAM cell is read by sensing the magnetic state (resistance level) of the MTJ element through a sense current flowing top to bottom through the cell in a current perpendicular to plane (CPP) configuration. During a write operation, information is written to the MRAM cell by changing the magnetic state in the free layer to an appropriate one by generating external magnetic fields as a result of applying bit line and word line currents in two crossing conductive lines, either above or below the MTJ element. In certain MRAM architectures, the top electrode or the bottom electrode participates in both read and write operations.
One indication of good device performance is a high magnetoresistive (TMR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the maximum change in resistance observed by changing the magnetic state of the free layer. In order to achieve desirable properties such as a specific junction resistance x area (RA) value, a high dR/R value, and a high breakdown voltage (Vb), it is necessary to have a smooth tunnel barrier layer that is promoted by a smooth and densely packed growth, such as a <111> texture for the AFM layer, pinned layer, and seed layer. Although a high RA value of about 10000 ohm-μm2 is acceptable for a large area (A), RA should be relatively small (<1000 ohm-μm2) for smaller areas. Otherwise, R would be too high to match the resistivity of the transistor which is connected to the MTJ. In addition to MRAM applications, an MTJ element with a thinner tunnel barrier layer to give a very low RA (<5 ohms-μm2) may be employed in TMR sensor head applications. Other desirable magnetic properties for an MTJ are a small interlayer coupling field (Hin) between the pinned layer and free layer, and a strong exchange coupling field (Hex) between the AFM layer and pinned layer is important to maintain the pinned layer magnetization in a certain direction.
One concern with a conventional Ta/NiCr (or Ta/NiFeCr) composite seed layer is that the growth of NiCr on Ta is sensitive to the Ta surface condition and depends on whether the Ta is an α-phase or β-phase material and if the Ta is partially oxidized. An improved configuration in a composite seed layer is necessary to provide a more consistent growth for the upper portion of the seed layer. The improved seed layer should also result in smoother layers in the MTJ stack and thereby generate a TMR device with higher performance capability for advanced applications that require high dR/R, low RA values, and low pin dispersion.
During a routine search of the prior art, the following four related patents were found. In U.S. Pat. No. 7,123,453, a magnetoresistive element having an exchange coupling film comprised of a NiFeHf seed layer and an underlying layer made of at least one element selected from Cr, Rh, Ta, Hf, Nb, Zr, and Ti, or a Ni alloy of one of the aforementioned elements is disclosed. U.S. Pat. No. 7,092,222 describes the same underlying layer as above but a NiFeCr seed layer with a high Cr content is employed to improve the wettability of the seed layer surface. U.S. Pat. Nos. 7,077,936 and 7,063,904 also relate to an exchange coupling film wherein a seed layer has an underlying layer made of at least one element selected from Cr, Rh, Ta, Hf, Nb, Zr, and Ti. However, the prior art does not teach a specific composition for an underlying layer comprising more than one element or whether or not the underlayer is an alloy.